Galaxies and galactic nuclei

Contents

ALMA can make high-resolution, large-scale, fully-sampled images of the molecular gas in galaxies, and it can map in detail the main dynamical components of all galaxy types in various mass ranges, such as spiral galaxies, elliptical galaxies, dwarf galaxies, and satellite galaxies. These maps will probe both parsec and kpc scales that are needed to explore the relationship between star formation, gas density and gas kinematics. The small-scale structure of the molecular component will clarify the mechanisms of starbursts/AGNs in galaxies, and the associated feedback processes, such as outflows of molecular gas, bubbles and winds.

The high-resolution images from ALMA allow us to study individual molecular clouds in nearby galaxies, including the Magellanic clouds, which are known to have lower metallicity and dust content, and have a star formation rate per unit area that is about 10 times larger than the solar neighborhood. ALMA can also be used to determine the masses and kinematics of optically obscured galactic nuclei with a resolution of a few parsecs and image the distributions of a variety of molecules.

Our own Galactic center provides a unique opportunity to study at very high spatial resolution the physical processes occurring in a galactic nucleus, in particular the nature of the molecular cloud population and the physical phenomena occurring in the vicinity of a massive black hole. The central region of our galaxy hosts a crowded environment with strong shear, magnetic fields and frequent cloud-cloud collisions. At the dynamical center lies Sgr A*, a strong radio continuum source and the best black hole candidate in the known Universe. ALMA observations will be essential to understand the nature of the ISM in the Galactic center and its star forming properties. Polarization measurements will also be extremely important in establishing the magnetic field geometry within the molecular gas – strong fields are thought to permeate much of the nuclear region.

Starbursts and AGN

Bolatto et al. (2013) discovered a starburst driven wind in the nearby starburst galaxy NGC 253 based on ALMA CO J=1-0 observations. The inferred molecular outflow rate is 3–9 Msolar/yr, implying a ratio of mass-outflow rate to star-formation rate of ~ 1–3. This suggests that the star formation activity in the galaxy is regulated by the starburst-driven wind and will therefore determine the final stellar content.

Figure 2.1: ALMA CO J=1-0 image of outflowing gas in NGC 253. Color represents the brightness of the emission (red is faint, purple is bright). The bright central lane, colored in green to purple, is the densest concentration of molecular gas, associated with the bar in this galaxy. The fainter (red) pillars of emission perpendicular to it correspond to ejected molecular gas. Credit: Alberto Bolatto, U. Maryland.

Fathi et al. (2013) analyzed the ALMA Band 7 data of NGC 1097, a Seyfert 1 galaxy that is known to contain a central AGN and a starburst ring. The ALMA HCN(4-3) data indicate that dense gas is streaming down to 40pc distance from the supermassive blackhole in this galaxy. The dense gas is confined to a very thin disk, and they derive a dense gas inflow rate of 0.09 Msolar/yr at 40 pc radius. Izumi et al. (2013) used the HCN, HCO+ and CS lines to characterize the physical condition of gas, and found that the high-J lines (J = 4–3 and 3–2) are emitted from dense (104.5 cm−3 ≤ n(H2) ≤106 cm-3) and warm (70 K ≤ Tkin ≤ 550 K) regions. They also suggest that the observed enhanced HCN emission arises from “high temperature chemistry” rather than pure gas phase PDR/XDR chemistry.

A well known Seyfert 2 galaxy NGC 1068 was observed in ALMA Band 7 and 9 by García-Burillo et al. (2014; see Figure 2.2), with an aim to investigate the molecular fuelling and the feedback processes in this galaxy. They find that the CO(3-2) emission is distributed throughout the starburst ring and the bar/inter-bar region, but the dense gas tracers (HCO+, HCN, CS) are mainly found near the r~200pc circum-nuclear disk. From their kinematical analysis, they suggest a molecular outflow of dM/dt ~ 63 Msolar/yr from the circum-nuclear disk region. The same galaxy was observed in ALMA Band 3 by Takano et al. (2014), who find that the SO, HC3N, and CH3CN molecules are concentrated in the circum-nuclear disk, the CS and CH3OH molecules are distributed in both the circumnuclear disk and the starburst ring, and 13CO and C18O are distributed along the starburst ring. NGC 1068 was observed more recently at high resolution in CO J=6-5 and 432 micron continuum by García-Burillo et al. (2016), resolving the circum-nuclear disk for the first time.

Merging galaxies and ULIRGs

The star formation rate in ultraluminous infrared galaxies (ULIRGs) is typically 10-100 times higher per unit mass of ISM than quiescent disk galaxies. As the closest ULIRG, Arp 220 has been frequently studied at all wavelengths to understand what drives the high star formation rate. Arp 220 contains two galactic nuclei separated by 412 pc. Scoville et al. (2017; see Figure 2.3) obtained ALMA images of the central region of Arp 220 at 37 pc resolution to resolve the internal gas distribution and kinematics of both galactic nuclei for the first time.

Galactic Center

Using the Science Verification data, Yusef-Zadeh et al. (2013; see Figure 2.4) studied the SiO emission near the central region of the Milky Way. They find from the ALMA observations that the interior of the circumnuclear molecular ring is not completely filled with ionized gas, as they found 11 clumps of molecular gas within 0.6 pc of Sgr A*. Based on the observed SiO spectra they suggest some of the clumps have protostellar outflows, indicating star formation may be ongoing close to the Galactic Center.

Figure 2.4: A combined ALMA and Very Large Array (VLA) image of the Galactic Center. The supermassive black hole is marked by its traditional symbol Sgr A*. The red and blue areas, taken with ALMA, map the presence of SiO, an indicator of star formation. The blue areas have the highest velocities, blasting out at 150-200 km/s. The green region, imaged with the VLA, traces hot gas around the black hole and corresponds to an area 3.5 x 4.5 light-years. Credit: Yusef-Zadeh et al. (2013), ALMA (ESO, NAOJ, NRAO), NRAO/AUI/NSF.